The present invention relates to a cutting tool comprising                at least one indexable insert having a top chip breaking surface, a bottom mounting surface and a conical clearance surface extending between said top and bottom surfaces and intersecting said top surface to form therewith a cutting edge with a substantially circular extension, said insert comprising at least one flat lower portion such that a cross-section in a plane through the indexable insert parallel to the bottom mounting surface intersects both said flat lower portion and said conical clearance surface, said indexable insert having a centre,        a holder comprising at least one holder member having a bottom surface forming a seat for supporting said bottom surface of said insert and an upstanding wall forming part of a cone configured to form a lateral support for said insert, said holder member comprising at least one flat lower portion such that bordering edges of the holder member when seen in the plane of a said insert received in the holder member parallel to the bottom surface contain both said flat lower portion and said upstanding wall, and        means for securing said insert to said holder, said upstanding holder wall and said insert clearance surface are designed so as to enable securing of said insert to said holder in a finite number of fixed relative positions,as well as an insert holder for such a cutting tool.        
The invention relates to cutting tools for chip removing machining in general, such as for example turning or milling, but since it is particularly directed to milling in the form of difficult roughing machining, such as of turbine blades, such a cutting tool for milling will hereinafter mainly be described for illuminating, but accordingly not restricting the invention thereto.
The holder of a cutting tool of this type may have a plurality of holder members each holding a said insert for alternatively or simultaneously machining a work piece by the cutting edges of the inserts.
The respective insert will then be securely held in the holder by securing means in a said fixed relative position until the part of the cutting edge used in the machining operation is worn out or otherwise fails, such as is broken, whereupon said securing means is released for moving the insert with respect to the holder for putting it in a new position in the holder, i.e. indexed, for securing the insert in such a new fixed relative position with respect to the holder for utilising another part of the cutting edge of the insert.
Important properties of such a cutting tool are high stability of the seat of the insert in the holder member in a said fixed relative position, i.e. firmly clamping of the insert to said holder in this position, efficient heat dissipation from the insert and easiness to carry out the indexing operation, especially to find a new said fixed relative position. The two properties first mentioned are of particularly vital importance when the working tool is designed for machining of work pieces being difficult to machine for any reason, which means that considerable forces will be applied to the insert and the generation of heat will be at a high level.
A cutting tool of the type defined in introduction is known through for example U.S. Pat. No. 5,346,336. The upstanding holder wall of a holder member of that cutting tool is formed by two flat sides or parts making an obtuse angle with each other, and these two flat sides co-operate with two mutually spaced flat lower portions in the lower part of the clearance surface of the insert for defining a said fixed relative position when the insert is secured in the holder member by tightening a screw. Thus, contact between the insert and the holder member is established mainly only through said flat lower insert portions bearing on said flat holder wall sides. Although this cutting tool provides a stability of the seat of the insert in the holder member and an ability to dissipate heat from the insert which may be acceptable for some type of machining, it may be desired to improve these properties as well as the property of easiness and reliability of indexing.
The object of the present invention is to provide a cutting tool of the type defined in the introduction being improved with respect to at least one of the properties mentioned above compared with such cutting tools already known.
This object is according to the invention obtained by providing such a cutting tool which is further characterized in that in each said relative position in said plane a first distance between the centre and the lower portion of the insert is less than a second distance between the centre and the lower portion of the holder member, which in turn is less than a third distance between the centre and the conical clearance surface and a curved portion of the upstanding wall laterally supporting said conical clearance surface.
The fact that said first distance is less than said second distance means that when the insert is secured in a said fixed relative position with the bottom surface thereof resting on the bottom surface of the holder member there will be a gap between the lower portion of the upstanding holder wall and the corresponding lower portion of the insert facing the lower portion first mentioned. This means that larger tolerances when manufacturing the insert and the holder member may be tolerated and still a perfect fit of these with respect to each other be obtained then would there be no such gap between said lower portions. This also means that it gets easier to obtain a real perfect tight abutment of said conical clearance surface on said curved portion, since there is no need of a perfect match of said lower portions with respect to each other. This contributes to efficient heat dissipation from insert to holder and prevents chips or dirt to penetrate between said curved portion and conical clearance surface.
According to an embodiment of the invention an upper part of said upstanding holder wall makes substantially the same angle with said bottom surface of the holder member as an angle made by an upper part of said insert clearance surface with said bottom surface of the insert for tight abutment of said upper part of the insert to said upper part of the holder member over the entire circumferential extension of said upstanding holder wall when the insert is secured to said holder.
By designing the holder member of the holder to have an upper part corresponding to the upper part of an insert to be held therein for obtaining said tight abutment of these upper parts over the entire circumferential extension of the upstanding holder wall a perfect mutual contact of the insert clearance surface and the upstanding holder wall is obtained with no possibility for chips or other dirt to penetrate therebetween and disturb this contact. This means that heat dissipation from the insert to the holder will be very efficient. Furthermore, this tight abutment over the entire circumferential extension of the upstanding holder wall contributes to a high stability of the seat of the insert in the holder member.
According to another embodiment of the invention the holder member is configured to receive said insert with said at least one lower portion of the upstanding holder wall facing said at least one lower portion of the insert clearance surface in each said fixed relative position, and said at least one lower portion of the upstanding holder wall and said at least one lower portion of the insert clearance surface are designed to have a gap therebetween over the entire extension of these lower portions when said upper part of the insert clearance surface bears tightly upon the upper part of the upstanding holder wall and these lower portions are facing each other in each said fixed relative position.
According to another embodiment of the invention said at least one lower portion of the upstanding holder wall makes a smaller obtuse angle with respect to the bottom surface of the holder member then the obtuse angle made by said at least one lower portion of the insert clearance surface so as to define said gap, and that the difference the obtuse angles made by said at least one lower portion of the upstanding holder wall with the bottom surface of the holder and said at least one lower portion of the insert clearance surface with the bottom surface of the insert is 1°-20°, especially 1.5°-5°. This is a suitable possible way to obtaining said preferred gap resulting in no mutual contact of the lower portion of the upstanding holder wall and the corresponding lower portion of the insert facing the lower portion first mentioned, but other favourable ways of obtaining this are also envisaged, such as making a local recess in the upstanding holder wall below said upper part thereof and then have the lower portion of this holder wall extending at a substantially constant distance to the lower portion of the insert. When choosing the option of different said obtuse angles the magnitude of the angle difference may be chosen so that it is ensured that there is no risk of abutment while preferably keeping this difference as small as possible for facilitating proper definition of said fixed relative positions.
According to another embodiment of the invention said at least one lower portion of the upstanding holder wall extends over a restricted part of the circumferential extension of the upstanding holder wall, and the insert has a plurality of said lower portions spaced apart over the circumferential extension of the insert clearance surface and each extending over a corresponding restricted part of the circumferential extension of the insert clearance surface so as to define said fixed relative positions when facing said at least one lower portion of the upstanding holder wall for indexing between such positions. This constitutes a preferred and efficient way of defining said fixed relative positions for indexing of the insert.
According to another embodiment of the invention said at least one lower portion of the upstanding holder wall makes a larger obtuse angle, such as 3°-15° or 5°-10° larger, with said bottom surface of the holder than made by said upper part of said upstanding holder wall with this bottom surface. It may by this by simple means be obtained that only said upper part of the upstanding holder wall bears upon the insert clearance surface where a said lower portion is located.
According to another embodiment of the invention said upstanding holder wall has more than one said lower portion, and the mutual angular spacing of these lower portions is the same as a mutual angular spacing of corresponding lower portions in the lower part of said insert clearance surface. By having more than one said lower portion of the upstanding holder wall said fixed relative positions may be very reliably defined and easily obtained.
According to another embodiment of the invention said lower portions of the upstanding holder wall are separated by lower upstanding holder wall parts making a smaller obtuse angle, such as 3°-15° or 5°-10° smaller, with said bottom surface of the holder member than the obtuse angle made by said lower portions of that wall with said bottom surface, and said lower portions of the insert are separated by a lower insert clearance surface part making a smaller obtuse angle with said bottom surface of the insert than made by said lower portions of the insert with this bottom surface. The presence of these lower upstanding holder wall parts separating the lower portions of the upstanding holder wall makes indexing very easy to carry out, since it will be impossible to put the insert in another position than in a said fixed relative position with respect to the holder member, since it will only fit with the bottom surface onto the bottom surface of the holder member with the lower portions thereof facing lower portions of the upstanding holder wall. These features make it impossible to put the insert and secure it otherwise than in a said fixed relative position in the holder member.
According to another embodiment of the invention said lower portions of the upstanding holder wall and of the insert clearance surface are flat.
According to another embodiment of the invention the upstanding holder wall has two said lower portions, and said insert has suitably 2, 3, 4, 5, 6, 7 or 8 said lower portions of the insert clearance surface. The number of such lower portions may depend upon for which type of machining the cutting tool is intended, which decides the number of preferred possible fixed relative positions for indexing, and 4 is a typical such number.
According to another embodiment of the invention said upper part of said upstanding holder wall extends over 10-50% of the vertical extension of the upstanding holder wall from the top thereof to said bottom surface and preferably over 10-30% of this extension. This results in a suitable size of the area of perfect contact between the insert and the holder member.
According to another embodiment of the invention said upstanding holder-wall has a circumferential extension of 90°-270°, preferably 120°-200°. An increasing such circumferential extension may increase the stability of the seat of the insert in the holder member and also the heat dissipation ability.
According to another embodiment of the invention the cutting tool comprises a plurality of said inserts and holder members distributed on the holder around an intended axis of rotation of the holder, which may be preferred when the cutting tool is used for milling.
The invention also relates to an insert holder for holding at least one insert in a cutting tool, and this insert holder is designed as the holder defined in any of the embodiments described above of a cutting tool according to the present invention. The advantages and advantageous features of such a holder appear clearly from the above discussion of the cutting tool according to the present invention.
Further advantages as advantageous features of the invention appear from the following description.